The invention relates to a method for exciting a gas dynamic CO.sub.2 laser, especially at high stagnation temperatures and to a laser device for performing such excitation method. The gaseous laser medium resulting from the combustion in a combustion chamber expands through a Laval nozzle and flows through the resonator at supersonic speeds, whereby the gaseous laser medium is available in the inversion state.
To satisfy the foregoing requirements, it is known to combust gaseous or liquid fuels such as CO, H.sub.2, CH.sub.4, C.sub.6 H.sub.6, C.sub.2 N.sub.2, with O.sub.2 or N.sub.2 O, whereby the combustion gases are thinned down substantially by the addition of N.sub.2 so that the CO.sub.2 content does not exceed a concentration of about 5 to 10%. It is known that for operation at combustion chamber pressures above about 50 bar and Laval nozzle configurations with neck diameters of about 0.1 to 1.0mm, a low CO.sub.2 concentration is necessary in order to establish a population inversion in the resonator. Such population inversion requires that a deactivation of the thermally excited vibration level, which is coupled with the upper laser level, is avoided as the laser medium flows through the Laval nozzle. Such deactivation could be the result of collision relaxation which thus is to be avoided. Reference is made in this context to an article entitled "PULSED GASDYNAMIC LASERS" by Walter H. Christiansen, AIAA Paper No. 71- 572, AIAA 4th Fluid and Plasma Dynamics Conference, Palo Alto, Calif., June 21-23, 1971.
However, where the operation takes place under the known operating conditions with chemical reactions resulting in a combustion product comprising N.sub.2 and a CO.sub.2 content less than about 10% as well as a desired H.sub.2 O content below about 5%, the stagnation temperatures T that may be realized are also below 2500.degree. K. Thus, the theoretically obtainable laser power related to the mass throughput is also limited. This laser power increases proportionally to 1/(exp(3380/T)-1) according to an article entitled "GASDYNAMIC LASERS" by Edward T. Gerry, published in IEEE Spectrum No. 7, page 51, 1970. As a result, the operating costs rise accordingly and a respectively large expense for the storage and conveying of the required large fuel quantities cannot be avoided. Such large expense applies especially to the N.sub.2 component of the prior art method if this component is stored in its gaseous state. This is so, because the gaseous N.sub.2 component requires relatively large and heavy containers maintained under high pressure which in turn requires expensive compressors of the large volume type with a respectively large power requirement.
It is further known to admix to the fuel mixture liquid nitrogen having a temperature of 77.degree. K. For this purpose it is also necessary to make the nitrogen N.sub.2 available in expensive cryogenic containers. Due to the low temperature of the nitrogen N.sub.2, this method achieves only combustion chamber temparatures which are about 200.degree. K below those temperatures which may actually be achieved in the above admixing of gaseous N.sub.2 to the fuel. As a result, the required mass consumption is even higher and so are the overall costs.
In prior art methods, the stagnation temperature is positively limited to T .ltorsim. 2500.degree. K, which in turn requires the use of Laval nozzles having an area ratio .ltorsim. 100 in order to obtain a gas temperature of about 300.degree. K in the resonator which is advantageous for the laser amplification. Prior art types of operation further require that the entrance or neck diameters of the Laval nozzles are small, namely about 0.1 to 1.0mm, whereby it becomes necessary in connection with the structure of larger laser devices to construct the expansion nozzle device from a plurality of very small Laval nozzles arranged in parallel. In such a device it is hard to cool the nozzles and the manufacturing costs are high.